The invention relates generally to electrochemical fuel cells and, more particularly, to a fuel cell with an electrode having catalyst disposed within the volume between its major surfaces. A method and apparatus for reducing reactant crossover from one electrode to the other in an electrochemical fuel cell is provided.
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Fluid reactants are supplied to a pair of electrodes which are in contact with and separated by an electrolyte. The electrolyte may be a solid or a liquid (supported liquid matrix). Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly comprising a solid ionomer or ion-exchange membrane disposed between two planar electrodes. The electrodes typically comprise an electrode substrate and an electrocatalyst layer disposed upon one major surface of the electrode substrate. The electrode substrate typically comprises a sheet of porous, electrically conductive material, such as carbon fiber paper or carbon cloth. The layer of electrocatalyst is typically in the form of finely comminuted metal, typically platinum, and is disposed on the surface of the electrode substrate at the interface with the membrane electrolyte in order to induce the desired electrochemical reaction. In a single cell, the electrodes are electrically coupled to provide a path for conducting electrons between the electrodes through an external load.
At the anode, the fuel moves through the porous anode substrate and is oxidized at the anode electrocatalyst layer. At the cathode, the oxidant moves through the porous cathode substrate and is reduced at the cathode electrocatalyst layer.
Electrochemical fuel cells most commonly employ gaseous fuels and oxidants, for example, those operating on molecular hydrogen as the fuel and oxygen in air or a carrier gas (or substantially pure oxygen) as the oxidant. The anode and cathode reactions in hydrogen/oxygen fuel cells are shown in the following equations:
Anode reaction: H2xe2x86x922H++2exe2x88x92
Cathode reaction: 1/2O2+2H++2exe2x88x92xe2x86x92H2O
The catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion-exchange membrane facilitates the migration of protons from the anode to the cathode. In addition to conducting protons, the membrane isolates the hydrogen-containing gaseous fuel stream from the oxygen-containing gaseous oxidant stream. At the cathode electrocatalyst layer, oxygen reacts with the protons that have crossed the membrane to form water as the reaction product.
In liquid feed electrochemical fuel cells, one or more of the reactants is introduced to the electrocatalyst in the liquid form. Examples of electrochemical fuel cells which can be operated with a liquid fuel feed are those employing a lower alcohol, most commonly methanol, as the fuel supplied to the anode (so-called xe2x80x9cdirect methanolxe2x80x9d fuel cells) and oxygen to the cathode. In fuel cells of this type the reaction at the anode produces protons, as in the hydrogen/oxygen fuel cell described above, however the protons (along with carbon dioxide) arise from the oxidation of methanol. An electrocatalyst promotes the methanol oxidation at the anode. The methanol may alternatively be supplied to the anode as vapor, but it is generally advantageous to supply the methanol to the anode as a liquid, preferably as an aqueous solution. In some situations, an acidic aqueous methanol solution is the preferred feed to the anode. The anode and cathode reactions in a direct methanol fuel cell are shown in the following equations:
Anode reaction: CH3OH+H2Oxe2x86x926H++CO2+6exe2x88x92
Cathode reaction: 3/2O2+6H++6exe2x88x92xe2x86x923H2O 
Overall reaction: CH3OH+3/2O2xe2x86x92CO2+2H2O
The protons formed at the anode electrocatalyst migrate through the ion-exchange membrane from the anode to the cathode, and at the cathode electrocatalyst layer, the oxidant reacts with the protons to form water.
Other non-alcohol fuels may be used in liquid feed fuel cells, for example formic acid. The oxidant may also be supplied as a liquid, for example, as an organic fluid with a high oxygen concentration (see U.S. Pat. No. 5,185,218), or as a hydrogen peroxide solution.
In electrochemical fuel cells employing liquid or solid electrolytes and gaseous or liquid reactant streams, crossover of a reactant from one electrode to the other is generally undesirable. Reactant crossover may occur if the electrolyte is permeable to the reactant, that is, some of a reactant introduced at a first electrode of the fuel cell may pass through the electrolyte to the second electrode, instead of reacting at the first electrode. Reactant crossover typically causes a decrease in both reactant utilization efficiency and fuel cell performance. Fuel cell performance is defined as the voltage output from the cell at a given current density or vice versa; the higher the voltage at a given current density or the higher the current density at a given voltage, the better the performance.
In solid polymer electrochemical fuel cells the ion-exchange membrane may be permeable to one or more of the reactants. For example, ion-exchange membranes typically employed in solid polymer electrochemical fuel cells are permeable to methanol, thus methanol which contacts the membrane prior to participating in the oxidation reaction can cross over to the cathode side. Diffusion of methanol fuel from the anode to the cathode leads to a reduction in fuel utilization efficiency and to performance losses (see, for example, S. Surampudi et al., Journal of Power Sources, vol. 47, 377-385 (1994) and C. Pu et al., Journal of the Electrochemical Society, vol. 142, L119-120 (1995)).
Fuel utilization efficiency losses arise from methanol diffusion away from the anode because some of the methanol which would otherwise participate in the oxidation reaction at the anode and supply electrons to do work through the external circuit is lost. Methanol arriving at the cathode may be lost through vaporization into the oxidant stream, or may be oxidized at the cathode electrocatalyst, consuming oxidant, as follows:
CH3OH+3/2O2xe2x86x92CO2+2H2O
Methanol diffusion to the cathode may lead to a decrease in fuel cell performance. The oxidation of methanol at the cathode reduces the concentration of oxygen at the electrocatalyst and may affect access of the oxidant to the electrocatalyst (mass transport issues). Further, depending upon the nature of the cathode electrocatalyst and the oxidant supply, the electrocatalyst may be poisoned by methanol oxidation products, or sintered by the methanol oxidation reaction.
The electrode structures presently used in direct methanol solid polymer fuel cells were originally developed for hydrogen/oxygen fuel cells. The anode electrocatalyst which promotes the oxidation of methanol to produce protons is typically provided as a thin layer adjacent to the ion-exchange membrane (see U.S. Pat. Nos. 5,132,193 and 5,409,785 and European Patent Publication No. 0090358). The anode electrocatalyst layer is typically applied as a coating to one major surface of a sheet of porous, electrically conductive sheet material or to one surface of the ion-exchange membrane. This provides a limited reaction zone in which the methanol can be oxidized before contacting the membrane electrolyte. Thus, with this type of electrode, the methanol concentration at the anode-electrolyte interface will typically be high.
Reactant crossover may be substantially eliminated if a reactant introduced to a first major surface of a fuel cell electrode is substantially completely reacted on contacting the second major surface of the electrode. In this case essentially no unreacted reactant would be available to pass from the second surface through the electrolyte to the other electrode. As described herein, this may be accomplished by ensuring that the reactant contacts sufficient catalyst so that it is substantially completely reacted before it contacts the second surface of the electrode.
It is therefore an object of the invention to provide an electrochemical fuel cell in which crossover of a reactant from one electrode to the other is reduced.
It is a further object of the invention to provide a solid polymer electrochemical fuel cell in which a reactant is substantially completely reacted before it contacts the membrane electrolyte.
It is a still further object of the invention to provide a direct methanol solid polymer fuel cell in which methanol crossover from the anode to the cathode is reduced.
Another object of the invention is to provide a method for reducing reactant crossover in an electrochemical fuel cell.
The above and other objects are achieved by an electrochemical fuel cell in which an electrode has catalyst disposed within the volume thereof. The electrochemical fuel cell comprises:
(a) a first electrode, the first electrode having first and second oppositely facing major surfaces, the first electrode comprising at least one layer of porous material and a sufficient quantity of catalyst disposed within the volume of the electrode between the major surfaces so that a reactant in a fluid introduced to the first major surface of the first electrode is substantially completely reacted upon contacting the second major surface of the first electrode;
(b) a second electrode;
(c) an electrolyte interposed between the second major surface of the first electrode and the second electrode.
In a preferred aspect of an electrochemical fuel cell the first and second oppositely facing major surfaces of the first electrode are planar.
In one embodiment the catalyst is distributed substantially uniformly within the volume between the first and second oppositely facing major surfaces of the first electrode. In an alternative embodiment the catalyst is distributed nonuniformly, such as, for example, in discrete layers or regions. The at least one layer of porous material may optionally comprise a plurality of stacked layers, and may optionally further comprise carbon particles. Suitable carbon particles include acetylene blacks, furnace blacks and graphite particles.
The electrolyte may be a liquid or a solid. With a liquid electrolyte a porous, electrically non-conductive separator is typically employed between the two electrodes. In a preferred embodiment the electrochemical fuel cell is a solid polymer fuel cell and the electrolyte comprises an ion-exchange membrane. The at least one layer of porous material is preferably electrically conductive and in a further embodiment comprises a proton conductor. Preferred porous materials comprise electrically conductive sheet material such as carbon fiber paper or carbon cloth. In an alternative aspect the at least one layer of porous material comprises carbon particles and a polymeric binder.
The fluid in which the reactant is introduced may be a liquid or a gas.
In a preferred embodiment of a liquid feed electrochemical fuel cell, the first electrode is an anode and the reactant comprises an alcohol, preferably methanol. In this case the catalyst promotes the oxidation of methanol. The fluid in which the methanol is introduced preferably comprises water and may optionally further comprise acid. In a preferred embodiment of a liquid feed direct methanol fuel cell the at least one layer of porous material comprises a plurality of carbon fiber paper layers each of the layers having two oppositely facing major surfaces, wherein catalyst is disposed on at least one major surface of each of the layers.
In any of the above embodiments of a solid polymer electrochemical fuel cell, catalyst may also be applied to the surface of the ion-exchange membrane.
The electrode has first and second oppositely facing major surfaces and comprises:
(a) at least one layer of porous material;
(b) a sufficient quantity of catalyst disposed within the volume of the electrode between the major surfaces so that a reactant in a fluid introduced to the first major surface of the electrode is substantially completely reacted upon contacting the second major surface of the electrode.
In a preferred aspect of an electrode the first and second oppositely facing major surfaces of the electrode are planar.
In the electrode, the porous material acts as a carrier for the catalyst, and is preferably liquid and gas permeable, to allow gas or liquid feed reactant to penetrate it and to allow gaseous products to escape. The porous material may be electrically non-conductive or preferably electrically conductive. The electrode, as a whole, is electrically conductive, however non-conductive porous material incorporating sufficient catalyst and/or other electrically conductive material to render the electrode electrically conductive may be used. The porous material may, for example, be one or more layers of electrically conductive particles, such as carbon particles, and a polymeric binder. The preferred porous material includes a sheet material which is self-supporting and has structural integrity, thus providing structural support for the adjacent ion-exchange membrane in solid polymer fuel cells. Suitable non-conductive sheet materials include expanded polytetrafluoroethylene and glass fiber matting, which preferably have electrically conductive particles, such as, for example, carbon particles associated therewith. Suitable electrically conductive sheet materials include carbon aerogel, carbon foam, carbon sponge, expanded metals and reticulated metals. Preferred sheet materials include carbon fiber paper and carbon cloth. The electrode may optionally incorporate structures to facilitate movement of gaseous products away from the electrode, for example, channels, grooves and layers or regions of different porosities.
In general, in electrochemical fuel cells, higher catalyst loadings lead to improved performance. The electrode structures described herein facilitate use of higher catalyst loadings.
In a method of substantially completely reacting a reactant in a fluid within a first electrode of an electrochemical fuel cell, the first electrode having first and second oppositely facing major surfaces, the reactant introduced at the first major surface, the first electrode comprising at least one layer of porous material and a catalyst, and the fuel cell further comprising a second electrode and an electrolyte interposed between the second major surface of the first electrode and the second electrode, the method comprises:
disposing a sufficient quantity of the catalyst within the volume of the first electrode between the major surfaces thereof such that the reactant is substantially completely reacted upon contacting the second major surface.